PROJECT SUMMARY/ABSTRACT Despite major advances in diagnosis, treatment, and prognosis for patients with breast cancer over the past several decades, the need for improved therapeutic strategies for metastatic disease is critical. Metastatic disease accounts for greater than 99% of all breast cancer-related deaths, and is often refractory to standard therapies, including targeted therapies for estrogen receptor (ER) positive and human epidermal growth factor receptor 2 (HER2) positive disease and chemotherapy for triple-negative breast cancer (TNBC). TNBC is a particularly aggressive subtype with a propensity to metastasize to vital organs including the brain, liver, and lung, and treatment is limited to toxic chemotherapeutics which are often ineffective in slowing disease progression or preventing recurrence. To improve outcomes for these patients, it is imperative to develop more effective, less toxic treatment regimens that incorporate targeted therapies. The androgen receptor (AR), a current therapeutic target in metastatic prostate cancer, is expressed in a significant percentage of breast tumors and has shown promise as a therapeutic target in several preclinical and clinical breast cancer studies. However, given the tendency of single-drug regimens to result in therapeutic resistance and relapse, it is important to develop combination regimens to treat advanced TNBC. Furthermore, the role of AR in metastatic disease is not well-characterized. The proposed studies will use clinically-relevant patient-derived xenograft (PDX) models in addition to breast cancer cell lines to investigate AR as a target for the treatment of metastatic breast cancer, and to screen for FDA-approved drugs that are effective in combination with AR inhibition. Preliminary studies have indicated that enzalutamide, an FDA-approved AR inhibitor, is cytotoxic to AR-positive TNBC cells in vitro. Preliminary analyses of clinical gene expression datasets revealed that AR expression varies based on intrinsic subtype, and that there is a significant negative correlation between AR expression level and metastasis-free survival in ER-negative disease. These studies suggest that AR-targeted therapy may be suitable for a subset of TNBC patients, who currently are pharmacologically limited to chemotherapy. The aims of the proposed study are: 1) to evaluate the effects of AR inhibition on the viability of AR-positive TNBC cells in vitro; 2) to identify FDA-approved compounds that are effective in combination with AR-targeted therapy; and 3) to test the efficacy of combination regimens including AR inhibition in vivo in treating mammary tumors and metastases in the brain, liver and lung. Subsequent analysis of the pathways targeted by effective drug combinations using patient tumor gene expression datasets will help determine which combination regimens would be suitable for further preclinical development and eventual clinical testing. The data generated from these studies can potentially have a major impact on treatment decisions and outcomes for patients with advanced AR-positive TNBC.